1. Field of the Invention
The invention pertains to apparatus and processes used in treating paper, and more particularly to foam applicators used to apply foamed treating compositions to moving paper web, such as during papermaking and finishing operations.
2. Description of Background Information
The application of foamed treating compositions to substrates has been achieved using various techniques. Foam applicators used to apply foamed treating compositions to a substrate are described in U.S. Pat. No. 4,023,526 (Ashmus et al.). The Ashmus et al. patent describes foam applicators which may be selected from a broad range of nozzle configurations, which can be used to treat any porous substrate such as textile fabric or a non-woven material, paper, leather or wood veneer. The foam applicator described in the Ashmus et al. patent provides for the application of treating agent in a foamed composition enabling low wet pickup, i.e. reduced deposition of liquid, such as water, onto the treated substrate. The Ashmus et al. patent is directed primarily towards, and all the specific embodiments involve, treatment of textile fabric.
Textile fabrics are air permeable to a relatively high degree, which property facilitates the separation air from liquid in a foam stream at contact with the fabric for textile. In contrast, many paper materials such as unfinished writing papers, book papers, newsprint, linerboard, boxboard, containerboard, and the like, are substantially non-porous being relatively low in permeability. Such papers are also, in comparison to textiles, relatively low in absorbency and very low in absorbency rate of liquids. The relatively low level of absorbency and particularly the low rate of liquid absorbency presents serious difficulties in obtaining suitable treating agent distributions from foamed compositions, particularly at high rates of treatment, such as those used in commercial papermaking and finishing operations.
It has been discovered that foam applicators as broadly described in the Ashmus et al. patent are limited in their ability to provide for the uniform distribution of foamed treating composition onto substantially non-porous paper webs at high processing speeds.
Other techniques for applying foamed treating compositions to paper have been developed. U.S. Pat. No. 4,081,318 (Wietsma) describes the application of foamed treating compositions to paper webs passing through the screening area of paper making machines using suction to draw a foam on to the web. The Wietsma system is designed to apply the foam to the paper without touching it or compressing it in any way by direct mechanical contact. Another no-contact type of foam applicator is described in U.S. Pat. No. 4,348,251 (Pauls et al.). U.S. Pat. No. 4,158,076 (Wallsten) describes a process and apparatus for applying foamed treating compositions to paper web whereby uniform distribution of treating composition onto the paper is achieved using a foam application zone having an opening in, or upstream of, the area where foam contacts the paper.
There is therefore a need for a foam applicator and process for treating paper which provide for uniform application of a broad selection of paper treating agents using fluid treating compositions which may vary over a wide range of composition viscosity, actives concentration and the like, and be applicable to a variety of paper webs including non-porous paper, moving at relatively high processing speeds, such as in-process papermaking and finishing operations.